1. Field of the Invention
This invention relates to solid state relay circuits and, more particularly, to solid state relay circuits with overload and short circuit protection circuitry and additional circuitry to protect the circuit from rapidly occurring voltage changes caused by electromotive induction in the output circuitry.
2. History of the Prior Art
A great variety of solid state relay circuits have been developed which use a power semiconductor as the output circuit switching device. A major disadvantage of such circuits has been their sensitivity to current overload or short circuit which may destroy the switching device. For this reason, various circuitry has been devised to turn off the switching devices when overload currents or short circuit conditions occur. Examples of such circuits are disclosed in U.S. Pat. Ser. No. 4,581,540, entitled Circuitry Overload Protected Solid State Relay, Ciro Guajardo, issued April 8, 1986.
Although such circuits provide appropriate protection against current overloads and short circuits in the load circuitry, a major disadvantage of such circuits has been their sensitivity to voltage transients in the load circuitry. Such voltage transients may be transferred by the interterminal capacitance to the gate of the switching device and cause the momentary turn on of the switching device at an inopportune time. Often these voltages transients are due to electromotive induction in the output circuit so that they are continually recurring. It is therefore necessary to, essentially, immunize the output switching device against such voltage transients. This is especially difficult where the switching circuit includes silicon controlled rectifiers (SCRs) as a part of the current overload protection.
It is therefore an object of this invention to provide improved solid state relay circuits.
It is another object of this invention to provide solid state relay circuits which include current overload protection with circuitry for eliminating the response of the output switching device to voltage transients in the load circuit.
It is another object of this invention to provide solid state relay circuits incorporating circuitry for reducing the response of the output switching device to the conductive susceptibility of silicon controlled rectifiers (SCRs) when voltage transients occur in the load circuit.